1. Field of the Invention
The present invention relates to the design of semiconductor light-emitting devices. More specifically, the present invention relates to novel semiconductor light-emitting device structures with a metal support substrate.
2. Related Art
Solid-state lighting is expected to be the illumination wave of the future. High-brightness light-emitting diodes (HB-LEDs) are emerging in an increasing number of applications, from light source for display devices to light-bulb replacement for conventional lighting. Typically, cost, efficiency, and brightness are the three foremost metrics for determining the commercial viability of LEDs.
An LED produces light from an active region which is “sandwiched” between a positively-doped layer (p-type doped layer) and negatively-doped layer (n-type doped layer). When the LED is forward-biased, the carriers, which include holes from the p-type doped layer and electrons from the n-type doped layer, recombine in the active region. In direct band-gap materials, this recombination process releases energy in the form of photons, or light, whose wavelength corresponds to the energy band-gap of the material in the active region.
In recent years, an increasing demand has emerged for blue LED's. Blue LEDs are generally based on wide band-gap semiconductor materials, including nitride materials such as GaN. Successful epitaxial growth of nitride materials requires matching of the lattice constant and thermal-expansion coefficients of the substrate and epitaxial layers. Consequently, unconventional substrate materials, such as sapphire (Al2O3) and silicon carbide (SiC), are often necessary to achieve such matching. Since sapphire has low conductivity, an LED fabricated on such substrates often requires the electrodes to be on the same side of the substrate. However, this lateral-electrode configuration can reduce light-emitting efficiency, increase fabrication complexity, and limit heat dissipation during operation.
To overcome these limitations, researchers have been experimenting wafer-bonding techniques to construct vertical-electrode LEDs. During wafer bonding, a second support wafer is bonded to the top of the LED device structure, and the initial growth substrate on which the device is epitaxially formed is removed. The entire device is then “flipped” upside-down. The new support substrate can have high conductivity, and therefore can facilitate vertical electrode configurations.
However, wafer bonding often needs expensive bonding materials, such as gold, and can introduce mechanical defects in the bonding process which can lower device reliability and decrease production yield. Hence, what is needed is a device structure which facilitates vertical electrode configuration, is less costly and more reliable, and which can provide superior mechanical characteristics.